1. Field of the Invention
This invention relates generally to conveying apparatus, and more specifically to improved braking control for conveying apparatus.
2. Description of the Prior Art
Escalators are provided with an electrically released, mechanically applied brake capable of stopping an up or down traveling escalator with any load less than the design load of the brake.
In the down-operating mode, when a stop command is initiated, the escalator may begin to travel faster, if there is a sufficient load, unless the brake is energized simultaneously with the stop command. To avoid this situation, braking action should occur as soon as the power is removed from the escalator. With the escalator fully loaded in down operation and the brake applied continuously, it may still take several inches of travel to bring the escalator to a complete stop. With no load in down operation and the brake applied at the same instant as the stop signal, the escalator may stop in approximately 1.5 inches of travel. This deceleration is too rapid, and it would therefore be desirable to modulate the braking action in some manner so that the stopping distance is approximately the same for both the fully loaded and unloaded down traveling escalator. Likewise, it is desirable to obtain the same deceleration rate for all load conditions.
When the escalator is operating in the up mode, and the brake is energized when the stop is initiated, the escalator may stop within approximately 1.5 inches under any load condition from no load to full load. If the brake is not energized with the stop command, the escalator may stop after approximately 16 inches of travel for no load and about 4 inches for full load. If loaded, the escalator reverses unless the brake is applied. It is possible to use a flywheel to extend escalator travel in the up direction to obtain a smoother deceleration.
It is well known in the art of conveying apparatus that a more uniform deceleration rate may be obtained for different travel directions and loads by using a feedback scheme. In such a system, a reference signal representing the desired acceleration or deceleration (or speed) of the conveying apparatus is compared to a signal representing the actual acceleration or deceleration (or speed) thereof. The difference signal can then be used to control a motor, for an acceleration situation, or a brake, for a deceleration situation. Many examples of such systems exist in the art.
In vertical transportation systems, for example, U.S. Pat. No. 3,589,474 which is assigned to the assignee of the present invention, discloses a reference signal and means for comparing the reference signal with the actual elevator speed to control elevator deceleration. Similar systems are also available in horizontal transportation apparatus. For instance, U.S. Pat. No. 3,519,805, which is assigned to the same assignee as the present invention, discloses a closed-loop acceleration control system wherein the actual acceleration or deceleration of a vehicle is fed back for comparison with a desired vehicle acceleration or deceleration profile. The difference signal is used to control the stopping of the vehicle such that the passengers remain comfortable as the vehicle comes to a stop. U.S. Pat. No. 4,231,452, which is assigned to the assignee of the present invention, also discloses a similar feedback scheme as applied to escalators. The actual escalator speed is compared to a deceleration speed pattern and the difference signal is used to control the solenoid of a brake in an analog fashion. That is, the brake is applied and released gradually using mechanical springs and dashpots, electrically-modeled dashpots, or a feedback scheme for tracking a predetermined speed pattern. U.S. Pat. No. 3,701,414 discloses yet another similar system applied to conveyors. Here again, a reference signal is compared to a signal representing the speed of the conveyor to produce a command signal. The command signal is then used to decelerate the conveyor through a brake assembly.
One problem with the prior art braking and acceleration schemes discussed above is that the comparison technique provides an on/off (i.e., pulse-based or two state) braking or acceleration signal. It would be desirable to in some way modulate the braking or acceleration signal to allow gradual, instead of abrupt, application.